Foil bearings are used in a multiplicity of applications, particularly in high-speed applications, for example in thermal flow machines, turbochargers, compressors, etc.
Foil bearings are a special type of plain bearings. They are aerodynamic bearings which are designed for high rotational speeds. Foil bearings of the first generation consist of what is referred to as the top foil, the bump foil and the bearing back. In order to improve the stability of a shaft in said bearing back, a plurality of, typically three, prestressing elements (for example material strips with a rectangular cross section and the length of the bearing width, what are referred to as “shims”) distributed over the circumference are placed between the bearing back and the bump foil. Without prestressing elements of this type, the aerodynamic buildup of pressure only takes place at a point is the circumferential direction of the shaft, namely where the shaft is caused by an external force to lie against the top foil. With the prestressing elements, by contrast, the buildup of pressure takes place at, for example, three points on the circumference.
FIG. 3 shows a schematic view for explaining an example of a foil bearing 300 without prestressing elements.
The foil bearing 300 has a bearing back 101, a shaft 103, a foil arrangement 107 with a top foil 107b and with a bump foil 107a. The foil arrangement 107 is arranged in a gap 303 between the shaft 103 and the bearing back 101. In FIG. 3, the shaft. 103 lies against the top foil 107b only at one cross-sectional point. This gives rise to a gap 303 which narrows in the circumferential direction and in which the hydrodynamic or aerodynamic pressure can build up. The distribution of pressure in the foil bearing is sketched schematically with the curve 301.
FIG. 4 illustrates a further example of a foil bearing 400 with prestressing elements 401, 403, 405.
The foil bearing 400 has a bearing back 101, a shaft 103, a foil arrangement 407, 407′, 407″ each having a top foil 407b and a bump foil 407a. The foil arrangement 107 is arranged in a gap 409 between the shaft 103 and the bearing back 101. In order to stabilize the shaft, the prestressing elements 401, 403, 405 are additionally arranged in the gap.
It can be seen in FIG. 4 that, by means of the prestressing elements 401, 403, 405, narrowing gaps arise at a plurality of points over the bearing circumference between the shaft 103 and the top foil 407b. At each of said narrowing portion, an aerodynamic pressure builds up in each case, said aerodynamic pressure supporting the shaft and positioning the latter in the radial direction. The position and the height of the prestressing elements 401, 403, 405 define the geometry of the construction. However, after the foil arrangement 407, 407′, 407″ and the prestressing elements 401, 403, 405 are installed, the geometry of the bearing 400 can no longer be changed, the position and the strength of the pressure gradients are then only still dependent on the operating conditions (speed, viscosity of the medium and the temperature).
US 2004/0042691 A1 discloses a foil bearing with a plurality of foils which are in the shape of segments of a ring and are arranged in a gap between a shaft and a bearing back. The bearing back here is pierced by pins, wherein the length by which the pins project out of the bearing back can be set by a rotatable ring surrounding the bearing back.
US 2003/0118257 A1 shows a foil bearing with a multiplicity of piezoelectric actuators. The foil bearing comprises a housing and also a plurality of foils for supporting a rotating shaft. The piezoelectric actuators are designed to set the stiffness and the damping coefficient of the bearing.